Noise is a noisy, unpleasant sound. In particular, the noise of an airplane or car running on a highway often causes environmental problems.
Noise is actually defined as environmental pollution under the Environmental Basic Act and environmental quality standards are also in place (Environmental Basic Act (Law No. 91, 1993) Environmental Quality Standards for Noise under the provisions of paragraph 1 of Article 16).
In order to address noise, it is necessary to accurately measure and analyze noise. It is also extremely important to secure the accuracy and reliability of measured/analyzed values.
However, the noise to be measured varies in terms of time or space. Therefore, it is difficult to obtain an accurate measured value of the noise for a short period of time.
Accordingly, in order to address noise, it is desirable that the measurement of noise go on for a long time/long period of time and that the noise be analyzed based on the measured data.
When noise is actually measured, the best way is manned measurement: a skilled measurement member who has long experience in measurement techniques is permanently stationed in the fields and carries out measurement while grasping the situation. However, when the measurement continues for a long time/long period of time, the problems is the costs and burden of the measurement member.
Therefore, when the measurement of noise goes on for a long time/long period of time, automatic measurement is often used instead of a measurement member being permanently stationed in the fields: a measurement device is disposed to automatically measure.
As a conventional system of such an automatic measurement type, for example, there is a noise measurement management system that automatically measures noise levels and records actual sound as well as analyzes acquired noise data and easily reproduces the actual sound as disclosed in PTL 1 (which is referred to as Conventional Technique 1, hereinafter).
The noise measurement management system of Conventional Technique 1 is equipped with a function of measuring a noise level from audio input signals, a function of analyzing and storing noise level data, a function of displaying and printing a chart diagram of noise levels, and a function of recording actual sound whose noise level is greater than or equal to a predetermined threshold value. Thanks to such functions, it is possible to carry out automatic measurement.
In measuring noise, it is important to recognize (judge) whether the detected audio input signals are those to be measured.
In the case of the manned measurement where an experienced measurement member manually carries out measurement, this recognition is easy.
However, in the case of the automatic noise measurement, the recognition is difficult. Accordingly, it is necessary for a measurement member to manually adjust a recognition condition with the use of some artificial recognition condition.
In the noise measurement management system of Conventional Technique 1, typical dark noise (background noise) is simply separated from noise, and a certain constant dark noise level is set as a threshold value. The noise measurement management system is so configured as to judge noise (noise judgment) when an input audio signal is greater than the threshold value and store the noise.
According to Conventional Technique 1, a measurement member sets the dark noise level as a recognition condition.
In such a case where only the dark noise level is used as a recognition condition, the problem is that the reliability of a measured value is lowered.
As the one that uses an advanced noise recognition condition, what is disclosed in NPL 1 is a noise measurement method of setting the following condition as a recognition condition (which is referred to as Conventional Technique 2, hereinafter): a case where the frequency band of an input audio signal has a characteristic or a case where radio waves emanating from an airplane and noise are detected at the same time.
When the noise judgment method of Conventional Technique 2 is used, it does not take much time and effort to sort out data because the data to be acquired can be whittled down more than a process of simply acquiring all the data that is greater than or equal to a threshold value.
PTL 1: JP-A-2001-165763
NPL 1: Yamashita, Oohashi, “Ways and means to automatically measure aircraft noise in the fields,” Noise Control, The Institute of Noise Control Engineering/Japan, 2005, Vol. 29, No. 5, p. 363-367